An optical encoder measures positions, lengths, and displacements contactlessly. They are often used as a feedback loop for positioning in machine controls. This position sensor operates using light and a ruler with a reflection pattern. They position with low resolution, accurate to as little as 1.2 nm! Furthermore, this technology has low power consumption, making the optical encoder suitable for applications in vacuum environments.
You want to move an object within a machine. With an encoder, you know where the object is and the distance it needs to travel to reach the desired position. Does your application require low-resolution, high-speed position control?
The optical encoder can also be the solution for vacuum environments. Read all about the capabilities of this position sensor.
How does an optical encoder work?
Just like magnetic and inductive encoders, optical encoders also work with a ruler. In optical technology, the ruler consists of a pattern of reflectors, or the light-sensitive layer. When an object moves, the encoder is moved along the ruler. These rulers are available in both straight and curved versions.
Peter Verstappen, Account Manager at Sentech, explains: “The transmitter of an optical encoder is a light source. Often, this is a VCSEL, a piece of silicon that emits light. As soon as the ruler is illuminated by the transmitter, the pattern of reflectors reflects the light back to the receiver. This creates a wave motion, an amplitude. This motion is converted into a sine and a cosine, which are translated into a signal. This signal tells the motor control system what the actual position of an object is.”
Feedback loop on positioning
Encoders are often used as a feedback loop for the positioning in a machine. What does such a position control look like?
“A motor receives the command to move an object from A to B. To know where the object is located in the movement between A and B, you need a measurement. This measurement is the feedback loop to the motor control. If the motor control knows where the object is, the motor can deliver the right amount of power to bring the object to the desired position,” says Verstappen.
Incremental or absolute
Optical encoders are available as incremental and absolute systems. What is the difference between these two types of encoders?
Incremental encoders
An incremental encoder measures step by step. Specifically, they measure the change and direction of movement. Upon startup, an incremental system must 'home' to find the index – or the zero position.
Absolute encoders
An absolute encoder provides the actual position directly. This type of encoder does not need to 'home'. The system interprets every position as a unique signal.
Compared to incremental encoders, absolute encoders are more complex and have a higher latency value, resulting in a delay in data transmission.
If component size plays a major role, precise placement is of great importance. Placement speed is also crucial in this regard. This allows you to offer finished products at competitive prices.
5 benefits of an optical encoder
- Very accurateWith a resolution of up to 1.2 nm, optical encoders are one of the most accurate measurement systems in the field of positioning.
- Small construction typeDue to its compact form factor, the optical encoder is easy to integrate into compact machines. For example, housings measuring 9 x 7 x 1.2 mm are available.
- High speedsThanks to their advanced technology, optical encoders are suitable for high-speed applications. From 3 meters per second for entry-level systems to 10 meters per second for advanced systems.
- Suitable for vacuumBecause the light source of the optical encoder is a VCSEL, they require less power, causing them to generate much less heat. This also makes this technology suitable for vacuum environments.
- Insensitive to electronic interferenceThey are insensitive to external electronic interference. This is because optical systems operate with a balanced A and B signal. If this signal is disturbed, the differences between the A and B signals remain intact. As a result, a good signal remains.
When do you use an encoder?
Before selecting a specific type of encoder, it is important to know what an encoder does. An encoder measures positions, lengths, and displacements. They are often used to monitor a position, acting as a feedback loop on the positioning in a motor or machine control system.
Depending on the requirements and environmental factors, you determine which type of encoder best suits your application. For example, an inductive or magnetic encoder is better suited to a dirty environment.
Optical encoders from Celera Motion with MicroE technology are constructed from materials suitable for vacuum environments.
Position control in vacuum
In vacuum environments, there are no air molecules. Without air molecules, electronic systems cannot dissipate heat, or do so with difficulty. “For optical encoders with a lens, this means they overheat, causing them to break down quickly. Because the MicroE encoders contain a VCSEL, they consume less power and generate much less heat. This makes this encoder suitable for vacuum environments,” explains Verstappen.
Do you still want to use a system that gets too hot for a vacuum space? Connect the system to a conductive material. This is the only way to lose heat in a vacuum.
Measure displacements with extreme accuracy
An optical encoder measures displacements with an accuracy of up to 1.2 nm. However, the actual accuracy is determined by various factors, which also affects the price of the system. It is therefore important to know what level of accuracy your application truly requires.
Absolute and repeatability
Encoder systems are composed of two factors that determine the precision of your measurement: absolute accuracy and repeatability. When selecting a position sensor, it is good to know to what extent these factors are important for your application.
Absolute accuracy is the actual position in space, without prior calibration or calling up a reference signal. With repeatability, an index signal is present. When the machine starts, the encoder always moves to this preset point first. That point is always the same. Subsequently, all movements are made from the index.
Explanation:
With absolute accuracy, you move 10,000 mm, but how accurate is this 10,000 mm actually? In reality, that 10,000 mm could also be 10,004 mm. Repeatability is when you send the machine to the same position ten times. How much this position deviates from that position is the repeatability.
Tolerance field
Both absolute accuracy and repeatability have gradations in precision. The smaller the tolerance range, the more precise the measurement. Suppose the absolute accuracy is 10 µm and the repeatability is 1 µm. Then the encoder may deviate by a maximum of 1 µm from the reference point. This margin may deviate by 10 µm from the actual (absolute) position.
The degree of absolute accuracy has a major influence on the price of an encoder system. If repeatability is particularly important, you usually end up with a more affordable system. Often, high repeatability is sufficient for a reliable measurement.
Material ruler
Every material has its own coefficient of thermal expansion. For example, a metal ruler expands at high temperatures and during temperature changes. In contrast, glass has no coefficient of thermal expansion, making a glass ruler more reliable at higher temperatures.
Interpolating: an even lower resolution
Do you want a lower resolution? That is possible! By interpolating the signal, you can measure more steps per second. The original signal is divided into even smaller steps. How much you can interpolate depends on the capacity of your control system: the input frequency must be able to handle the pulse train. Ensure that the input frequency of the control system is higher than the output frequency of the encoder.
This is how you prevent a dirty encoder
Because optical systems work with light, dust and dirt are detrimental to the measurement results. To limit contamination, you can take this into account during the integration of the encoder.
The Account Manager explains: “In the design, there are options to ensure the system gets dirty less quickly. For example, you can mount it upside down or place it under a cover. Also consider fingerprints. In certain positions, the ruler will come into contact with fingers more quickly. Furthermore, optical systems are easy to clean. It is also advisable to leave space for this in the design.”
How do you select the right optical encoder?
Among optical systems, various variants are available. To select the right optical encoder, it is important to know which requirements and environmental factors are relevant. It is important to be specific in this regard.
If, for example, you know that a low resolution is important for your application, you also want to know exactly how accurate your system needs to be. And whether your system's control can handle it.
Map out which specifications are truly necessary for your application together with our sensor expert.